This invention relates generally to frequency shift keying demodulation circuitry and more particularly to frequency shift keying demodulation circuitry used with telecommunications equipment, such as a computer modem. In the art of FSK transmission, a unique tonal frequency is used to represent each piece of information. In order to represent a binary piece of information, two distinct tonal frequencies are necessary, one to represent a logical zero and one to represent a logical one. These two tones are known together as a frequency pair. In order to allow simultaneous full duplex communication, an originate frequency pair and an answer frequency pair have been defined by the prior art.
The key elements in the construction of an FSK demodulation circuit are a filter and a transformation and demodulation circuit used for transforming the incoming analog tonal frequency signal into a corresponding digital signal. In the FSK modems of the prior art, LC filters or other active filter circuits, which employ capacitors, resistors and an operational amplifier, are used. In the demodulation circuit of the prior art, a phase locked loop circuit is utilized. The filter and demodulation circuit must be accurate and have a narrow operating range in order to discriminate between both the zero frequency and the one frequency of the originating unit and the zero frequency and the one frequency of the answering unit.
In the filter units of the prior art, many inductive coils, resistors and capacitors of high precision are required. This yields a filter which is large in size and high in cost. Additionally, accurate adjustment of the filter is difficult and the aging of the components may cause the modem to fall out of specification. The demodulation circuit must also be large in size, as well as expensive, because of its need for high quality capacitors and resistors.
It is also noted that it may be desirable to have both the originating unit and answering unit transmitting simultaneously. This is known as full duplex communication. In such an instance, four distinct frequencies are required for FSK communication; a zero frequency and a one frequency for the originating units and a zero frequency and a one frequency for the answering unit. In order to have each modem operate as either an originating unit or an answering unit, it is necessary to have two complete sets of filters and demodulation circuits in each modem. Thus, the above mentioned disadvantages of size and cost are multiplied in a full duplex unit and complicated switching arrangements are necessary for full duplex operation.
It is the object of this invention to provide an FSK demodulation circuit wherein the filter and demodulation circuits are simple in construction, reliable and inexpensive to produce.